1. Field of the Invention
The present invention relates to an oblique observation attachment operable for use with microscopes providing visual inspection of planar subjects on an X-Y stage. Use of this attachment permits subjects to remain fully centered in the field of view of the microscope objective lens and in focus while the attachment is in use. The visual perspective of the observer shifts from a perspective normal to the subject through to a visual perspective oblique to the subject without repositioning of the subject or refocusing of the microscopes objective lens.
2. Description of the Prior Art
Inspection of populated planar products such as dual in-line packages (DIP's) of electronic circuitry requires the inspector to make observations with magnification from angles other than perpendicular to the focal plane of the circuit board subject. Careful inspection is required of solder joints interconnecting the pins of a chip package to a circuit board for defects and imperfections.
In the prior art, inspection of soldered populated planar products such as microelectronic hybrid circuit boards or printed wiring boards was a difficult task. As the latitudinal perspective changed during visual inspection under a microscope the optical distance between the microscope objective lens and the object of interest did not remain constant. Continual focusing and refocusing was required by the human hand of the microscope while manipulating the populated planar product beneath the objective lens of the microscope.
This practice of visual inspection also precluded the utilization of a preprogrammed automated inspection method. Consequently, all of such visual inspections made using a manual means were time consuming and inefficient.
The patent to L. T. Johnson entitled "Reflecting Microscope", U.S. Pat. No. 2,478,762 dated Aug. 9, 1949 discloses a reflecting microscope formed with a combination of an off-axis ellipsolidal mirror in combination with straight mirrors, the off-axis elliposolidal mirror having two focii shown in use with two plane mirrors. Light from an object is reflected from a first plane mirror as if it had come from a first focus on the ellipsoidal mirror. This light is reflected towards a second focus on the ellipsoidal mirror but the reflected light is intercepted by a second plane mirror and reflected back to the first focus of the ellipsoidal mirror. Multiple reflections between the two planar mirrors and the ellipsoidal mirror result in the light eventually passing through an aperture in the second planar mirror, and finally, the objective lens of the microscope itself to the awaiting human eye.
The patent to L. G. Margulis et al. U.S. Pat. No. 3,185,927 dated May 25, 1965 entitled "Probe Instrument for Inspecting Semiconductor Wafers Including Means for Marking Defective Zones" discloses a testing mechanism for manipulatively probing semiconductor wafer slabs having a plurality of regularly disposed individual indicia thereon whereby the electrical characteristics of the individual chips or components can be measured and inspected prior to dicing. A microscope is utilized in conjunction with the preferred embodiment of the Margulis invention preferably oriented so that the microscope has a 45.degree. angle existing between the axis of the objective lens and the top of the pedestal which holds the semiconductor slab.
The patent to M. Clerc entitled "Optical Scanning Device for Rapid Spectroscopy", U.S. Pat. No. 3,537,795 dated Nov. 3, 1970, discloses an optical scanning device for rapid spectroscopy. In the preferred embodiment of the Clerc invention a first rotating mirror having two reflective surfaces receives a beam of dispersed energy for observational purposes. A variation on the preferred embodiment of the Clerc invention utilizes four fixed elliptical mirrors having a first common focus at the intersection of the plane with the axis of rotation of the rotating first mirror. The second focus for two of the elliptical mirrors at a second focal point and a third focal point for the remaining two elliptical mirrors will receive any illuminous ray coming from the first focal point of all four elliptical mirrors. This will occur when all optical paths are then equal due to the fundamental property of ellipsoids.
Finally, the U.S. patent to T. Togo entitled "Laser Optical Apparatus for Operations Under a Microscope", U.S. Pat. No. 4,091,814 dated May 30, 1978 discloses a laser optical apparatus for operations under a microscope, visible light from an object is reflected by a plane mirror and formed into parallel rays of light by a parabolic mirror. These rays of light pass by the opposite sides of the 45.degree. reflector into a magnification electro-optical system. A control means operable to rotate the plane reflector about the vertical axis by means of a first servo-motor and about the horizontal axis by a second servo-motor is also incorporated into this design.
The problem to be solved therefore is the problem of optically inspecting the soldered joints of a printed wiring board or hybrid micro-electronic assembly from a normal and oblique perspective without the use of the human hand to refocus the microscope or move the subject in the field of view.
Some commercially available hardware does address this problem, for example, the Wild Hierburgg limited attachment for vertical and oblique observation for M3/M5A/M7AM/M7S, stock No. 422482 has addressed this problem. However, this design has the following shortcomings:
The mirrors must be shuttled in for oblique observation. This increase in the length of the optical path is compensated by the divergent lens, however, this lens also diverges light from any coaxial illuminator rendering it inadequate. PA1 The mirrors can make oblique viewing possible only from one angle. PA1 A planar mirror pivotable about the Z axis through a first focal point. PA1 A plano-elliptical mirror whose focii are at two distance focal points. PA1 A rotating support ring operable to rotate about the Y axis. PA1 A retaining ring and supports for the pivoting planar mirror.
A solution to these problems is presented by the preferred embodiment of this invention, which would allow an inspector of printed circuit boards and hybrid microelectronic circuit chips to no longer use the human wrist and hand as a gimballed system. The possibility of a fully automated inspection station including intelligent vision systems would become more feasible.